It is known that precipitated silicas can be rendered waterproof with silicon oils in a dry state (see published German Patent Application DE-OS 26 28 975--U.S. Pat. Nos. 4,273,589 and 4,308,074). Such waterproofed precipitated silicas are used as defoaming silicas in detergents.
The precipitated silicas waterproofed according to the known method have the disadvantage that the silicon oil used as hydrophobing agent is not distributed uniformly on the surface of the silica.
There has been, therefore, a need for a method of producing precipitated silicas, waterproofed with silicon oil, which have a uniform distribution of the silicon oil.